Among the conventionally-known internal combustion engines equipped with a decompression mechanism is one disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2002-235516 (hereinafter referred to as “the relevant patent literature”), in which a pair of supporting protrusions each having an insertion hole, is formed on a gear of a camshaft and in which a decompression member and a biasing spring are supported on a support shaft inserted through the respective insertion holes of the supporting protrusions. The decompression member is pivotably supported on a portion of the support shaft located between the pair of supporting protrusions. Further, the camshaft has a guide recess formed in a side of its peripheral surface opposed to the decompression member. The decompression member can move appropriately by an actuating section of the decompression member being moved along the guide recess.
Further, in the internal combustion engine disclosed in the relevant literature, a biasing spring, which is provided on a portion of the support shaft between the pair of supporting protrusions, normally biases the actuating section of the decompression member toward an actuating position. The actuating section is kept in a state where it adjoins a cam of the cam shaft and slightly projects beyond the cam, so that a push rod is slightly raised by the actuating section to keep an exhaust valve in a slightly-opened position so as to allow a starting or activating operation of the internal combustion engine to be performed in an appropriate manner. Once the internal combustion engine reaches a predetermined number of rotations, the decompression member moves, by centrifugal force, to a retracted position remote from the cam so that the push rod is no longer raised by the actuating section. In this way, the exhaust valve and an intake valve of the internal combustion engine can be opened and closed appropriately, with the result that the internal combustion engine can be driven in an appropriate manner.
However, because the biasing spring of the decompression mechanism extends to be located outside the pair of supporting protrusions, it is difficult to reduce the size of, or downsize, the decompression mechanism disclosed in the relevant patent literature; in this respect, the internal combustion engine disclosed in the relevant patent literature has a room for improvement. Further, in the decompression mechanism disclosed in the relevant patent literature, the guide recess is formed in one side of the outer peripheral surface of the camshaft for permitting appropriate movement of the decompression member. The presence of such a guide recess in the one side of the camshaft would make it difficult to keep smooth rotation of the camshaft; in this respect, the internal combustion engine disclosed in the relevant patent literature has another room for improvement.